Thanks for trying this - I'm a bit busy today, but that was going to be my next step; clean install of Linux in a VM.
I already did this; several times! It must be *some* setting or bad config file somewhere in my system, but it *only* affects KSP1.4.x. It's very very strange.
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Update:
SOLVED!
By default, KDE is setup to use a 101-key keyboard layout. My physical keyboard is a 105-key UK keyboard.
Changing the keyboard layout in KDE from 101 to 105 fixed it in KSP1.4.3.
Looks like Unity 2017 and/or KSP 1.4 changed the way how keys are processed considering earlier versions of KSP worked just fine.
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Update 2:
Really Solved (?)
It wasn't the physical layout; it was the fact that I had multiple keyboard maps configured to let me switch between languages. Removing one makes KSP work. Unity bug or KSP bug?
micha's post in (SOLVED) KSP1.4.3 Linux x64 Keyboard "letter" keys not working was marked as the answer